Planarization of optical substrates

ABSTRACT

Planarization of defects in laser mirror and other optical component manufacture is disclosed. The planarization is performed by first depositing a relatively thick planarization layer, then carrying out a sequential deposition and etch process. The technique takes advantage of the non-uniform material removal rate as a function of etchant incident angle, and effectively buries the inclusion in a thick film with a near planar top surface. The process enables faster, more reliable manufacture of a non-defective high fluence multilayer mirror particularly suitable for high energy laser applications.

REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. provisional patentapplication entitled: “Planarization of Multilayer Optical CoatingDefects,” filed Oct. 12, 2012, as Ser. No. 61/713,332, the contents ofwhich are incorporated herein by reference.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH AND DEVELOPMENT

The United States Government has rights in this invention pursuant toContract No, DE-AC52-07NA27344 between the United States Department ofEnergy and Lawrence Livermore National Security, LLC, for the operationof Lawrence Livermore National Laboratory.

BACKGROUND OF THE INVENTION

This invention relates to techniques for the manufacture of largeoptics, for example, as used in lasers for inertial confinement fusionpower plants, and in particular to the manufacture of mirrors for lasersfor use in such fusion power plants. The invention, however, generallyhas applicability to the planarization of optical substrates to overcomedefects in such substrates used for laser mirror coatings and defectscaused during the coating deposition process.

The National Ignition Facility (NIF) is a laser-based inertialconfinement fusion research machine located at the Lawrence LivermoreNational Laboratory (LLNL) in Livermore, Calif. NIF uses lasers to heatand compress a capsule of deuterium and tritium (DT) fuel to thetemperatures and pressures to cause a nuclear fusion reaction. In NIF abank of 192 lasers fires a hohlraum holding the capsule. The lasers usedin NIF are large, extremely powerful lasers, producing beams on theorder of a foot square.

Inertial confinement fusion power plants using the technology now beingdeveloped at NIF have been proposed. The equipment, systems and supportnecessary for the deployment of such a fusion power plant are now beinginvestigated and designed at LLNL. In the indirect drive approach toinertial confinement fusion (often “ICF” herein) proposed for such powerplants, hohlraums, each with a capsule containing the DT fuel, areinjected into a fusion chamber. As they arrive at the center of thechamber, the “targets” are fired upon by a bank of lasers. The hohlraumabsorbs and re-radiates the energy of the laser beams striking theinside of the hohlraum as x-rays onto the fuel capsule. This causes theouter surface of the fuel capsule to ablate, compressing and heating theDT fuel to cause a fusion reaction.

The lasers used in such a system operate at high energies withconcomitant heat and energy demands imposed on the components of thelaser. As such they impose unique design requirements on the opticalcomponents within the laser. Of particular concern here is thatmultilayer optical coatings are laser fluence limited by smallinclusions on the substrate or imbedded within the coatings. Theseinclusions are created by micron-sized particulates on the opticalcomponent substrate. The particulates result from imperfect substratecleaning, contamination during transport of the substrates aftercleaning to the coating apparatus, as well as other causes. The geometryof these inclusions and the interference nature of multilayer coatingscan lead to extremely high light intensification around the defect, thuscausing the defect to have a much lower laser resistance than thesurrounding non-defective multilayer coating.

Various approaches have been tried to minimize defects in smalleroptical components. In particular, for extreme ultraviolet lithography,small nanometer-sized contaminants are detrimental to the functionalityof the resulting masks. A variety of approaches have been tried, somewith success in addressing this issue. See, e.g., “A Silicon-Based,Sequential Coat-and-Etch Process to Fabricate Nearly Perfect SubstrateSurfaces,” Mirkarimi et al., Journal of Nanoscience and Nanotechnology,July, 2005, and “Advancing the ion beam thin-film planarization processfor the smoothing of substrate particles,” Mirkarimi et al.,Microelectronic Engineering 77 (2005) 369-381. Each of thesepublications describes techniques for mitigating surface imperfectionssuch as pits or particles in the coatings for extreme ultravioletlithography masks. The approaches described in the articles, however,address defects which are much smaller than those of concern here. Forexample, the techniques described in these articles address defects onthe order of a few tens of nanometers in depth, as opposed to themicron-sized defects problematic with the optical and near infraredcoatings. Furthermore, the materials used in the processes described inthese articles, primarily silicon, are highly absorbent of energies atthe wavelengths of the laser light. As such they cannot be employed forcomponents in which wavelengths in this range are used. Finally, theprimary concern addressed in these prior art approaches is one ofassuring reflectivity and surface flatness for mask transfer. Here, incontrast, the primary issue relates to energy concentrations around thedefects within the coating.

What is needed is a technique for mitigating nodular defects ofapproximately micron size in a manner to enable use of opticalcomponents that include such defects in laser or optical applications.

BRIEF SUMMARY OF THE INVENTION

The planarization of substrate defects to allow the deposition of adefect-free coatings has applications to high power lasers, both withinlarge laser projects such as at the National Ignition Facility, as wellas for components for smaller optics within commercial laser systems.The technology also has potential applications to low roughness coatingsfor experiments such as the Laser Interferometer Gravitational-WaveObservatory (LIGO) gravity wave detection program.

Optical multilayer coatings are fluence-limited by imbedded nodular(convex) defects. The geometry of these nodular defects, combined withthe interference nature of multilayer coatings, leads to lightintensification within these defects, thus initiating laser damage,particularly in large optics with high power lasers. To solve thisproblem, considerable research has addressed the source of the defects.Improvements such as semi-automated cleaning systems, e.g. manualcleaning followed by ultrasonic cleaning, have reduced substratecontamination. Clean rooms have reduced contamination arising frommovement of an optical component within a facility, as well as loadingof the component into the coating chamber. Load lock systems have alsoreduced particulates from transport and coating chamber pump-down.Switching from oxide to metallic coating materials has reducedparticulates created during deposition processes. Filtration techniquessuch as velocity filters (rotary vane filters) have also been employedfor some deposition processes to reduce particulates arriving on thesubstrates during deposition. Although substantial improvements havebeen observed in particle reductions, defects on the substrate stillexist, particularly for large aperture (meter class) optical thin films.

Because it is virtually impossible to prevent all defects on such largeoptical components, this invention focusses not on defect removal, buton defect mitigation. Here the defect is buried within a thickplanarization film upon which the multilayer coating is deposited.Alternatively the planarization process can occur during the multilayercoating deposition, ultimately providing a mirror coating with minimaldefect-induced light intensification. In one embodiment the multilayercoating is performed by sequential deposition and preferential etchingsteps. In particular, because ion beam etching is more effective at anangle to the surface, as opposed to perpendicular to the surface, thedome over a defect will etch more efficiently than the surroundingplanar coating. By repeated deposition and etching, a multi-layer filmcan be grown over the defect that effectively buries the defect, andprovides a planar top surface. Once the desired magnitude of planarityis achieved, any desired further coatings can be deposited on top of theoptic. Because the electric field at the bottom of the multilayercoating is low, the embedded defect will not initiate laser damage.

In a preferred embodiment, a method of making an optical component inwhich a substrate surface has at least about a one micron size nodulardefect includes steps of depositing a planarization layer over thedefect; depositing an ion beam etchable layer over the planarizationlayer; etching away a portion of the ion beam etchable layer; anddepositing a layer of a metal oxide over the ion beam etchable layer.Then the steps of depositing an ion beam etchable layer, etching away aportion of the ion beam etchable layer, and depositing a layer of metaloxide are repeated until the nodular defect is reduced in size thedesired amount.

In a preferred embodiment, the optical component is a laser mirror; theplanarization layer and the ion beam etchable layer each comprisesilicon dioxide (SiO₂); and the metal oxide comprises hafnium dioxide(HfO₂). Also useful as a planarization layer are hafnium dioxide (HfO₂),tantala (Ta₂O₅) or zirconia (ZrO₂). In the preferred embodiment theplanarization layer is thicker than the defect. The alternating layerspreferably comprise an oxide layer having a high coefficient ofrefraction which is optically suitable for the expected wavelengths.Also preferably, after deposition, e.g. by ion beam technology, theetchable layer is etched back about half its thickness after eachdeposition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating fluence demands placed upon high-powerlaser mirrors;

FIGS. 2 a-2 c illustrate the effect of an embedded micron sizedparticle;

FIG. 3 further illustrates light intensification caused by embeddednodules;

FIGS. 4 a-4 b illustrate a planarization process for addressing at leastabout micron-sized defects;

FIGS. 5 a-5 b illustrate use of a planarization layer;

FIG. 6 illustrates deposition and etching equipment;

FIG. 7 illustrates determination of planarization layer thickness;

FIGS. 8 a-8 d illustrate experiments performed on various size nodulesand planarizing layers; and

FIGS. 9 a-9 b and 10 a-10 b present further experimental results.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a diagram illustrating the increasing demands being placedupon laser mirrors in high-power applications displayed in terms offluence. Fluence, as discussed here, is the radiative flux integratedover time. On the horizontal axis are various lasers, with correspondingmirror fluence represented on the vertical axis. As shown, mirrorfluence rose from about 5 Joules per square centimeter (J/cm²) for theShiva laser (circa 1977) to 12 J/cm² for the Nova laser (circa 1984) to22 J/cm² for the present 1.8 MJ NIF laser (circa 2009). In futuregenerations of lasers, mirror fluence is expected to be on the order of65 J/cm² for a 3 MJ NIF laser. With ICF-based fusion power, it isexpected that mirror laser resistance will routinely exceed 100 J/cm².

Classically laser mirror and optic development has focused upon reducingthe number of defects introduced during deposition of coatings, tothereby mitigate laser damage. While this approach has been somewhatsuccessful for small optical components, with large optical components,for example, mirrors on the order of more than a foot square, theapproach has been unsuccessful.

FIG. 2 is a diagram illustrating the effect of an embedded micron sizedparticle in the multilayer coating of the laser mirror. To differentiatebetween these embedded micron sized particles which create a convexdefect, and scratches (or pits) which create a concave defect in thecoating, we refer to the convex defects as nodules.

As illustrated by FIG. 2 a, nodular coating defects on a substrate growradially in a parabolic manner as a function of film thickness. Thesenodular defects cause light intensification within multilayer mirroroptical coatings as illustrated by FIG. 2 b. The result of this lightintensification is the laser damage threshold of optical multilayermirror coatings is reduced by these defects. In particular, when themirror is subjected to powerful laser light, the resulting heat andenergy concentration causes expulsion of the particle and creation of adamage pit, as shown in FIG. 2 c.

FIG. 3 further illustrates the light intensification caused by embeddednodules as compared to a mirror lacking such embedded nodules. Notealmost a factor of 35 intensity enhancement (and thus likely resultingdamage) caused by a 2 μm diameter nodule. The three cases, u_(—)0,te_(—)45, and tm_(—)45 represent normal (0 degree) incidence, 45 degreeincidence at “S” polarization and 45 degree incidence at “P”polarization respectively.

FIGS. 4 a illustrates the effect of a micron sized nodule on planarityof a mirror. As shown in FIG. 4 a, a substrate 10 has a nodular defect15 present on its upper surface. A series of layers 20, e.g. amultilayer coating, are illustrated as having been deposited over thesurface. The figure illustrates the change in planarity caused by thedefect as additional layers are applied. As shown, the additional layersincrease the width 24 of the nodule as more and more layers aredeposited over it. At the same time the height 26 of the nodule remainsessentially constant as the additional layers are deposited.

FIG. 4 b illustrates one embodiment of a process according to thisinvention. As shown, the nodular defect 15 is effectively buried byinitial deposition of a planarization layer 30, followed by thesequential deposition of the layers to form a multilayer coating,typically a mirror. In effect, the planarization layer prevents thegrowth of the nodular defect on the substrate, resulting in a moreplanar upper surface. The planarizing effect of the process isexaggerated in FIG. 4 b, however, actual data from tests performed arepresented below.

The choice of materials for the layers illustrated in FIG. 4 b issomewhat arbitrary. The planarization layer is preferably a layer thattends to flow around the nodular defect, embedding the defect in thelayer. Our preferred material is silicon dioxide (SiO₂). Other possiblechoices for the planarization layer are hafnium dioxide (HfO₂), tantala(Ta₂O₅) or zirconia (ZrO₂).

Above the planarization layer, the alternating layers are preferablysilicon dioxide (SiO₂) and hafnium dioxide (HfO₂). Other materials canbe substituted for the silicon dioxide layer provided those materialshave the preferential etching characteristics of silicon dioxide, i.e.their surfaces etch more rapidly at an angle to the etching process thanif the layers are perpendicular to the etching. Other suitable materialsfor the high index of refraction layer are other optical oxide materialssuch as titanium dioxide (TiO₂), aluminum oxide (Al₂O₃) and niobiumoxide (Nb₂O₅). More generally, materials which are suitable for use in amulti-layered mirror coating can be used.

In FIG. 4 b, the series of layers as described above with combinedthickness 35 have been deposited over a nodule. If after deposition ofthe preferably alternating silicon dioxide layer, that layer ispartially etched away, for example, using ion beam etching, a moreplanar structure results, Note that in FIG. 4 b, the differentialetching characteristics of an ion beam etch are not illustrated. Thateffect, however, is shown in FIGS. 5 and 8 below.

With ion beam etching, the etching rate is dependent upon the angle ofthe beam with respect to the substrate. The ion beam preferentiallyattacks the sloped portions of the layers overlying the nodule andetches those faster than the flatter portions surrounding the nodule.The end result is that the defect diameter will gradually collapse assequential steps of depositing and etching are performed. This effect ofpreferential etching is illustrated in FIG. 5 a. Stated another way,when sputtering a defect such as in FIG. 5 a with an ion beam normal tothe substrate, the rounded section of the nodular dome (the portionbetween the center of the dome and the angle θ) etches at a faster ratethan the non-defective film surrounding the nodule. A combination ofrepeated silicon dioxide (SiO₂) deposition and ion beam etching causesthe nodular defect radius and height to collapse. This effect is shownin FIG. 5 b. Note how the defect diameter collapses with the sequentialetching steps. After a sufficient number of cycles, the defect will becompletely embedded in a thick silicon dioxide layer with a planarsurface over the defect. Once this is accomplished, the multilayermirror is far more planar, or depending on the application, additionallayers to make the desired optical component can be deposited over thestructure.

The process described above, when used to form a laser mirror, isperformed using an ion beam sputtering system in a reactive environmentwith the targets containing the desired material for the layers,preferably silicon and hafnium in an oxygen ambient. FIG. 6schematically illustrates the equipment used for carrying out theseprocesses. Note that by use of a rotating table in a vacuum chamber,layers may be alternately deposited on the substrates and then partiallyremoved from them—as described above.

As discussed, prior to deposition of the multilayer coating, aplanarization layer is deposited. FIG. 7 illustrates a preferredapproach for determination of the initial planarization layer thickness.As shown in FIG. 7, a preferred thickness for the planarization layer isa function of the expected defect size. For spherical defects, a layer1.2 times the size of the defect will planarize the defect. Forcylindrical defect of layer 1.5 times its expected size is sufficient,and for a cube shaped defect, a layer 1.7 times its expected size isappropriate. More generally, this means that a 3 μm thick planarizationlayer will effectively smooth a 2.5 μm sized spherical defect, a 2 μmsized cylindrical defect, and a 1.76 μm sized cubic defect.

In our preferred embodiment, a planarization layer of appropriatethickness, typically a thick layer of silicon dioxide, is deposited.Other possible choices for the planarization layer are hafnium dioxide(HfO₂), tantala (Ta₂O₅), or Zirconia (ZrO₂). For implementing lasermirrors, the layers disposed on top the planarization layer arepreferably alternating layers of silicon dioxide and hafnium dioxide.After each layer of silicon dioxide is deposited, however, approximatelyhalf its thickness is etched away. This approach takes advantage of thepreferential etching characteristics of ion beam etching, as describedabove. Thus, for example, if a 2 μm thick layer of silicon dioxide isdeposited, preferably about 1 μm of that layer is etched away before thenext layer of hafnium dioxide is deposited. Similarly, if 4 μm ofsilicon dioxide are deposited, then about 2 μm are etched away, etc.After the ion beam etching process, another layer of hafnium dioxide isdeposited. Then another layer of silicon dioxide is deposited and etchedback. The process is repeated as many times as desired to obtain thenecessary planarity of the ultimate surface.

In the preceding explanation of the preferred embodiment, etching awayabout half of the layer of silicon dioxide is described. It will beappreciated that this is an approximation and that more or less silicondioxide can be removed from the surface. For example, if the processallows, or if the number of layers is higher, lesser amounts of thesilicon dioxide can be removed. Removing smaller amounts, e.g. a quarterof the layer deposited, will typically require more deposition andetching steps, while removing more than half the layer thickness willrequire fewer steps. Our experiments show satisfactory results ifbetween about 25% and about 75% of the layer thickness is removed.

The inventors here have performed experiments on various size nodules,various thicknesses of planarizing layers and various processes fordepositing and removing the overlying mirror layers. In theseexperiments pillars (or mesas) were intentionally formed on the surfaceof a substrate, and then those “nodular defects” were subjected to theplanarization techniques described here. The results of some of theseexperiments are illustrated in FIG. 8.

FIG. 8 a illustrates deposition of a multilayer coating when noplanarization layer is used and no etch back is performed. The threeillustrations in FIG. 8 a show the effect of a 1 μm wide pillar, a 2 μmwide pillar, and a 5 μm wide pillar on planarity of the upper surface ofthe substrate, with no planarization layer and no etch back. Notice howeven minor surface roughness in the substrate propagates through all ofthe overlying layers of silicon dioxide and hafnium dioxide, and how thedefect size increases with the number of layers.

FIG. 8 b illustrates our planarization process where an initialplanarization layer of silicon dioxide is deposited, and then sequentiallayers of silicon dioxide and hafnium dioxide are deposited. In thisexperiment 2 μm thick layers of silicon dioxide were deposited with halfthat thickness etched away before deposition of the hafnium dioxidelayer. Notice how the defect size is reduced in each of the 1 μm, 2 μmand 5 μm wide defect illustrations. FIGS. 8 c and 8 d illustratecorresponding experiments with various size defects, differentthicknesses of layers and different amounts of etch back. Of particularinterest, note that the 1 μm and 2 μm wide pillar defects are almostcompletely removed by the combination of the thick planarization layerand the repeated deposition and etch back steps. Even the 5 μm widepillar is dramatically reduced in its impact on the upper surface, asshown across the bottom row of the photographs in FIG. 8.

FIG. 8 also includes the fluence test results for the structure shown ineach photograph. These test results show the laser resistance to thesubstrate defects. Notice, for example, that the untreated 5 μm widepillar defect in the lower row of FIG. 8 a, when planarized using thetechniques described here, exhibits an increase in fluence from about 5J/cm² to greater than 100 J/cm²(See FIG. 8 d, lower photo), animprovement of more than a factor of 20.

FIG. 9 presents further experimental results. FIG. 9 illustrates resultsof a laser damage summary for 2 μm wide pillars with 10 ns pulse lengthand a 1064 nm wavelength laser. FIG. 9 a shows results for Spolarization, while FIG. 9 b shows results for P polarization. Noticethat in the circumstance where no planarization is performed, damageoccurs at fluences above 40 J/cm². Yet with 1 μm, 2 μm and 3 μm etchback of silicon dioxide layers initially deposited with thickness of 2μm, 4 μm, and 6 μm respectively, no laser damage occurs, even forfluences of 100 J/cm². The planarization fraction, is plotted on theright, and shown by the curve in each of FIGS. 9 a and 9 b. In each ofthe figures the circular points represent no damage, and the pointsmarked with an “x” represent damage. FIGS. 10 a and 10 b present similardata for 1 μm pillars. Again note the dramatic increase in tolerance tohigh laser fluences by virtue of the planarization processes describedhere.

The foregoing has been a description of preferred embodiments of theinvention. It will be appreciated that numerous details, materialcompositions, and the like, have been provided to explain the invention.The scope of the invention, however, is set forth in the appendedclaims.

What is claimed is:
 1. A method of making an optical component in which a substrate surface has at least a one micron size nodular defect comprising: depositing a planarization layer over the defect; depositing an ion beam etchable layer over the planarization layer; etching away a portion of the ion beam etchable layer; depositing a layer of a metal oxide over the ion beam etchable layer; and repeating the steps of depositing an ion beam etchable layer, etching away a portion of the ion beam etchable layer, and depositing a layer of metal oxide, until the nodular defect is reduced in size a predetermined amount.
 2. A method as in claim 1 wherein: the optical component comprises a mirror; the ion beam etchable layer comprises silicon dioxide (SiO₂); and the layer of metal comprises hafnium dioxide.
 3. A method as in claim 2 wherein the planarization layer comprises silicon dioxide (SiO₂),
 4. A method as in claim 2 wherein the planarization layer comprises hafnium dioxide (HfO₂).
 5. A method as in claim 1 wherein the planarization layer comprises one of tantala (Ta₂O₅) and zirconia (ZrO₂).
 6. A method as in claim 1 wherein the step of depositing an ion beam etchable layer comprises depositing at least a one micron thick layer of silicon dioxide.
 7. A method as in claim 6 wherein the step of etching away comprises removing a substantial amount of the layer of silicon dioxide.
 8. A method as in claim 7 wherein the step of etching away comprises removing about half of the layer of silicon dioxide.
 9. A method as in claim 1 wherein the step of depositing a layer of silicon dioxide comprises sputtering silicon dioxide.
 10. A method as in claim 9 wherein the step of depositing a layer of silicon dioxide comprises ion beam sputtering silicon dioxide.
 11. A method as in claim 1 wherein the step of etching away comprises ion beam etching.
 12. A method of making a laser mirror in which a mirror substrate has at least a one micron size nodular defect comprising: depositing a planarization layer over the mirror substrate and the nodular defect; depositing a layer of silicon dioxide over the planarization layer; etching away a portion of the layer of silicon dioxide; then depositing a layer of hafnium dioxide over the layer of silicon dioxide; and repeating the steps of depositing a layer of silicon dioxide, etching away a portion of the layer of silicon dioxide, and depositing a layer of hafnium dioxide until the nodular defect is reduced in size a predetermined amount.
 13. A method as in claim 12 wherein the step of depositing a layer of silicon dioxide comprises depositing a layer of controlled thickness.
 14. A method as in claim 13 wherein the step of etching away a portion comprises etching away about half of the thickness of the silicon dioxide.
 15. An optical component made by the process of claim
 12. 16. An optical component comprising: a substrate having at least an about one micron size nodule thereon; a planarization layer disposed across the substrate and over the nodule; and an alternating sequence of layers of silicon dioxide and hafnium dioxide disposed on the planarization layer.
 17. An optical component as in claim 17 wherein the planarization layer comprises silicon dioxide.
 18. An optical component as in claim 16 wherein the planarization layer is at least about 1.2 microns thick.
 19. An optical component as in claim 16 wherein the planarization layer is at least about 2 microns thick.
 20. An optical component as in claim 16 wherein the silicon dioxide layers comprise sputtered silicon dioxide and the hafnium dioxide layers comprise sputtered hafnium. 